The development of high-performance molecule-based electro-optic (EO) materials has been the focus of much current research. Such materials are of great scientific and technological interest not only for applications as diverse as optical telecommunications, signal processing, data storage, image reconstruction, logic technologies, and optical computing, but also for the fundamental understanding how matter interacts with light. The essential requirement for large bulk EO response is that the active component chromophore has a large microscopic molecular first hyperpolarizability tensor (β), and the quest for such chromophores has been a very active research field. To date, the vast majority of effective EO chromophores have been devised according to very similar design principles: planar conjugated π-electron systems end-capped with electron donor and acceptor (D, A) moieties. This design algorithm gives rise to a dominant intramolecular charge-transfer (ICT) transition from the ground state to first excited state and produces effective polarization along the π-conjugated axis. Considerable efforts have been directed toward the molecular engineering of such chromophore structures, and a variety of strategies has emerged within the framework of the classical “two-state model” for molecular hyperpolarizability. This simple model invokes a neutral ground state and a charge separated first excited state, where β is determined by the energy gap between the two states (ΔEge), the transition dipole moment (μge) between the two states, and the difference in the dipole moment between the two states (Δμge=μee−μgg) (eq 1).β=3Δμge(μge)2/(ΔEge)2  (1)
One approach, described in terms of “bond length alteration” (BLA), the difference between average single and double bond lengths in the conjugated chromophore core, argued that BLA, hence β, can be optimized by controlling the relative neutral and charge-separated contributions to the ground state via modifying D/A constituent strength, the polarity of the solvent, or the strength of an applied electric field. Another model, “auxiliary donors and acceptors”, correlates molecular hyperpolarizability with the electron density of the π-conjugation, arguing that electron excessive/deficient heterocycle bridges act as auxiliary donors/acceptors, and lead to substantial increases in β values. Directed by these strategies, the largest hyperpolarizabilities have, to date, been observed with protected polyene and/or multiple (including fused) thiophene ring-containing bridges (e.g., CLD and FTC), with the chromophore figures-of-merits, μβ (μ=the molecular dipole moment), as high as 35,000×10−48 esu being achieved.

Note that such strategies focus primarily on extensive planar π-conjugation, and such molecules are inherently structurally complex, complicating synthetic access, and introducing potential chemical, thermal, and photochemical frailties. Furthermore, extended conjugated systems typically introduce bathochromic shifts in optical excitation, thus eroding transparency at the near-IR working wavelengths for many photonic applications. Other β enhancement strategies have emerged recently, including multi-dimensional charge-transfer chromophores (e.g., HPEB and X-CHR), and a class of “right-hand-side” zwitterionic chromophores (e.g., PCTE and RHSC). These chromophores exhibit improved transparency and stability, but not significant enhancement in hyperpolarizability.
As suggested in the literature the β responses of all known organic EO chromophores, for reasons that are presently not clear, fall far short of the fundamental quantum limits by a factor of ˜10−3/2. As a result alternative paradigms for very large-β chromophores remains an ongoing concern in the art, with growing evidence that simple two-state systems are inadequate.
Twisted intramolecular charge-transfer (TICT) molecules have recently received considerable attention in the quest to understand their nonlinear optical response. In TICT mechanisms, rotation about a bond connecting conjugated D/A substituents can decouple the orbitals of the D/A groups. Nearly complete electron transfer can occur, strongly enhancing CT interactions and leading to large hyperpolarizabilities. However, electron transfer is induced upon optical excitation, but not observed in the ground state. Further, such structures tend to be complex and prone to thermal/oxidative/photochemical degradation.